Communication standards applied to communication systems in recent years include, for example, Division Multiple Access (W-CDMA), Long Term Evolution (LTE), Worldwide Interoperability for Microwave Access (WiMAX: IEEE802.16-2004, IEEE802.16e, etc.). Further, the communication standards may include Orthogonal Frequency Division Multiplexing Access (OFDMA).
The above-mentioned wireless communication systems require linear efficient RF power amplifiers for wideband signals like that can provide higher data transmission rates. Conventional power amplifiers (PAs) are normally designed for peak efficiency under maximum (peak) output power condition. Consequently, when the power is backed-off from its maximum point, efficiency of the power amplifier drops sharply. As a result, the mean amplifier efficiency is much lower than the efficiency at peak power level.
There are composite type power amplifiers including a plurality of power amplifiers (usually only two, rare three or more). The composite power amplifiers include Doherty amplifiers and LINC or outphasing power amplifiers. Such multi-way amplification schemes are able to provide a high efficiency in power back-off operation through active load modulation in their active devices.
The LINC power amplifier (outphasing power amplifier) is one of the promising techniques that can simultaneously achieve high linearity and high power efficiency. The LINC amplifier takes an envelope modulated bandpass waveform and resolves it into two phased modulated (PM) constant envelope signals, which are applied to highly efficient and highly nonlinear power amplifiers, whose outputs are summed (combined).
FIG. 1 is a diagram illustrating a simplified block diagram of a LINC power amplifier and FIG. 2 is a vector diagram representing constant envelope signals S1(t) and S2(t) and an outphasing signal e(t). Separation of bandpass waveform (band-limited source RF signal: bandpass signal) is executed by a signal component separator (SCS). A complex representation of the bandpass signal (referred to as input signal) may be written by a formula (1).[Formula (1)]s(t)=a(t)·eθ(t); 0<a(t)<Vm  (1)
Where a(t) is the real envelope and Θ(t) represents the original phase modulation in the input AM signal. Vm is the maximum amplitude of the signal s(t).
The input signal is separated by the SCS into two constant-envelopes PM signals S1(t) and S2(t) having equal envelopes and opposite modulated phase variations. These two RF signals S1(t) and S2(t) with modulated phase and constant amplitudes can be represented as vector as illustrated in FIG. 2. The signals S1(t) and S2(t) and a phase angle ψ is represented by formulas (2), (3) and (4) below. Where e(t) is a quadrature or outphasing signal and defined by formula (5).
                                          S            1                    ⁡                      (            t            )                          =                              s            ⁡                          (              t              )                                -                      ⅇ            ⁡                          (              t              )                                                          (        1        )                                                      S            2                    ⁡                      (            t            )                          =                              s            ⁡                          (              t              )                                +                      ⅇ            ⁡                          (              t              )                                                          (        2        )                                          ψ          ⁡                      (            t            )                          =                              cos                          -              1                                ⁡                      (                                                                            a                  ⁡                                      (                    t                    )                                                                                              V                m                                      )                                              (        3        )                                          ⅇ          ⁡                      (            t            )                          =                  j          ·                      s            ⁡                          (              t              )                                ·                                                                      V                  m                  2                                                                      a                    2                                    ⁡                                      (                    t                    )                                                              -              1                                                          (        4        )            
The signal S1(t) and S2(t) is amplified by two power amplifiers connected to the SCS individually, and the amplified signal of The signal S1(t) and S2(t) is sent to the power combiner. With the power combining, the in-phase signal components add together and the out-of-phase signal components cancel out. The resultant signal is the desired amplified replica of the original source signal (see formula (6)).s(t)=0.5(S1(t)+S2(t));  (6)
For more information, see U.S. Pat. No. 5,012,220, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-533947, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-518514, Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-213090, U.S. Patent Application Publication No. 2010/0244949, U.S. Pat. No. 7,184,723, U.S. Patent Application Publication No. 2010/0074367, “X. Zhang et al., “Gain/Phase Imbalance-Minimization Techniques for LINC Transmitters”, IEEE Trans On Microwave Theory And Techniques, Vol. 49, No. 12, December 2001”, “I. Hakala, “A 2.14-GHz Chireix Outphasing Transmitter”, IEEE Trans On Microwave Theory And Techniques, Vol. 53, No. 6 June 2005”, and “Lawrence F. A., “High Efficiency Linear Power Amplifier for Portable Communications Applications,” CSIC 2005 Digest Wireless Technology Research Laboratory Motorola Labs.”
However, one of the major disadvantages of the LINC power amplifier is that the possible loss in the power combining network compromises the power efficiency. For example, when a conventional_hybrid is used as the combiner, power of the outphasing signal e(t) turns into waste heat. An alternative combining approach, called Chireix, expands an area having high efficiency to a low power output level by shunt susceptance compensation (compensation reactance). Chireix may change an efficiency curve so as to adapt to different peak to average power ratio by variation of the compensation susceptance (compensation reactance).
FIG. 3 illustrates the power efficiency versus the output power with and without shunt susceptance compensation. The output power is normalized to the maximum value, as has the efficiency. As depicted in FIG. 3, the appropriate selection of the compensation susceptance can result in a significant boost in system efficiency, especially at the lower output power level. With small susceptance, good compensation is obtained at low output power level, but the efficiency at higher power levels is lower than that without compensation, for now the compensation susceptances become excessive.